For the detection or modulation of optical radiation, especially at infrared frequencies, devices based on intersubband or bound-to-continuum excitation of carriers in quantum wells have been disclosed in Applied Physics Letters, 50, pp. 273-275, Feb. 2, 1987 (Levine et al. I) and Applied Physics Letters, 53, pp. 296-298, July 25, 1988 (Levine et al. II). The former article discloses intersubband absorption and the latter article discloses bound to continuum absorption.
The devices described include a substrate supported layered structure having quantum wells between barrier layers. The devices have interleaved wide and narrow bandgap layers and are expediently implemented, e.g. by means of doped gallium arsenide well layers and aluminum gallium arsenide barrier layers on a gallium arsenide substrate. At least one narrow bandgap layer forms a quantum well. Suitable choices of layer thicknesses and compositions permit the devices to have peak absorption at any desired wavelength in the "atmospheric window" region extending from 8 to 14 micrometers. The devices are considered suitable for use in, e.g. focal-plane arrays, high-speed detectors, optical heterodyne receivers, and vertically integrated infrared spectrometers.
However, due to quantum mechanical selection rules, absorption in these devices depends upon the direction of the incident radiation with respect to the layered structure, and there is essentially zero absorption for radiation which is incident perpendicular to the layered structure. A waveguide configuration permits light absorption but is not easily implemented; grating couplers have been proposed to increase the absorption efficiency. For example, Applied Physics Letters, 47, pp. 1257-1259, Dec. 15, 1985, discloses the use of a grating to enhance the absorption efficiency. The grating, patterned GaAs covered by a metallic layer, was designed to convert incident radiation into evanescent modes with an electric field component perpendicular to the quantum well layers, thus permitting it to be absorbed. Additionally, Applied Physics Letters, 53, pp. 1027-1029, Sept. 19, 1988, describes a structure similar to that of the previous article, but which absorbs light also by diffracting the incident radiation back through the layered structure. The diffracted radiation is not normal to the quantum well layers and may thus be absorbed.